Nicole Joller

The aryl hydrocarbon receptor interacts with c-Maf to promote the differentiation of type 1 regulatory T cells induced by IL-27

Type 1 regulatory T cells (Tr1 cells ) that produce interleukin 10 (IL-10) are instrumental in the prevention of tissue inflammation, autoimmunity and graft-versus-host disease. The transcription factor c-Maf is essential for the induction of IL-10 by Tr1 cells, but the molecular mechanisms that lead to the development of these cells remain unclear. Here we show that the ligand-activated transcription factor aryl hydrocarbon receptor (AhR), which was induced by IL-27, acted in synergy with c-Maf to promote the development of Tr1 cells. After T cell activation under Tr1-skewing conditions, the AhR bound to c-Maf and promoted transactivation of the Il10 and Il21 promoters, which resulted in the generation of Tr1 cells and the amelioration of experimental autoimmune encephalomyelitis. Manipulating AhR signaling could therefore be beneficial in the resolution of excessive inflammatory responses.

Silencing MicroRNA-155 Ameliorates Experimental Autoimmune Encephalomyelitis

IFN-γ–producing Th1 and IL-17–producing Th17 cells are the key participants in various autoimmune diseases, including multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis (EAE). Although both of these T cell subsets are known to be regulated by specific transcription factors and cytokines, the role of microRNAs that control these two inflammatory T cell subsets and whether targeting microRNAs can have therapeutic effects are not known. In this study, we show that microRNA-155 (Mir-155) expression is elevated in CD4+ T cells during EAE, and Mir-155−/− mice had a delayed course and reduced severity of disease and less inflammation in the CNS. The attenuation of EAE in Mir-155−/− mice was associated with a decrease in Th1 and Th17 responses in the CNS and peripheral lymphoid organs. The T cell-intrinsic function of Mir-155−/− was demonstrated by the resistance of Mir-155−/− CD4+ T cell-repleted Rag-1−/− mice to EAE. Finally, we found that anti–Mir-155 treatment reduced clinical severity of EAE when given before and after the appearance of clinical symptoms. These findings demonstrate that Mir-155 confers susceptibility to EAE by affecting inflammatory T cell responses and identify Mir-155 as a new target for therapeutic intervention in multiple sclerosis.

Treg cells expressing the coinhibitory molecule TIGIT selectively inhibit proinflammatory Th1 and Th17 cell responses.

Foxp3(+) T regulatory (Treg) cells regulate immune responses and maintain self-tolerance. Recent work shows that Treg cells are comprised of many subpopulations with specialized regulatory functions. Here we identified Foxp3(+) T cells expressing the coinhibitory molecule TIGIT as a distinct Treg cell subset that specifically suppresses proinflammatory T helper 1 (Th1) and Th17 cell, but not Th2 cell responses. Transcriptional profiling characterized TIGIT(+) Treg cells as an activated Treg cell subset with high expression of Treg signature genes. Ligation of TIGIT on Treg cells induced expression of the effector molecule fibrinogen-like protein 2 (Fgl2), which promoted Treg-cell-mediated suppression of T effector cell proliferation. In addition, Fgl2 was necessary to prevent suppression of Th2 cytokine production in a model of allergic airway inflammation. TIGIT expression therefore identifies a Treg cell subset that demonstrates selectivity for suppression of Th1 and Th17 cell but not Th2 cell responses.

Cutting edge: TIGIT has T cell-intrinsic inhibitory functions.

Costimulatory molecules regulate the functional outcome of T cell activation, and disturbance of the balance between activating and inhibitory signals results in increased susceptibility to infection or the induction of autoimmunity. Similar to the well-characterized CD28/CTLA-4 costimulatory pathway, a newly emerging pathway consisting of CD226 and T cell Ig and ITIM domain (TIGIT) has been associated with susceptibility to multiple autoimmune diseases. In this study, we examined the role of the putative coinhibitory molecule TIGIT and show that loss of TIGIT in mice results in hyperproliferative T cell responses and increased susceptibility to autoimmunity. TIGIT is thought to indirectly inhibit T cell responses by the induction of tolerogenic dendritic cells. By generating an agonistic anti-TIGIT Ab, we demonstrate that TIGIT can inhibit T cell responses directly independent of APCs. Microarray analysis of T cells stimulated with agonistic anti-TIGIT Ab revealed that TIGIT can act directly on T cells by attenuating TCR-driven activation signals.

MyD88-Dependent IFN-γ Production by NK Cells Is Key for Control of Legionella pneumophila Infection

Legionella pneumophila (Lpn) is a ubiquitous Gram-negative bacterium in aquatic systems and an opportunistic intracellular pathogen in immunocompromised humans causing a severe pneumonia known as Legionnaires’ disease. Using a mouse model, we investigated molecular and cellular players in the innate immune response to infection with Lpn. We observed robust levels of inflammatory cytokines in the serum upon intranasal or i.v. infection with live, virulent Lpn, but not with inactivated or avirulent bacteria lacking the Icm/Dot type IV secretion system. Interestingly, Lpn-induced serum cytokines were readily detectable regardless of the capacity of Icm/Dot-proficient Lpn to replicate in host cells and the Lpn permissiveness of the host mice. We found NK cell-derived IFN-γ to be the key cytokine in the resolution of Lpn infection, whereas type I IFNs did not appear to play a major role in our model. Accordingly, NK cell-depleted or IFN-II-R-deficient mice carried severely increased bacterial burdens or failed to control Lpn infection, respectively. Besides the dependence of inflammatory cytokine induction on Lpn virulence, we also demonstrate a strict requirement of MyD88 for this process, suggesting the involvement of TLRs in the recognition of Lpn. However, screening of several TLR-deficient hosts did not reveal a master TLR responsible for the sensing of an Lpn infection, but provided evidence for either redundancy of individual TLRs in Lpn recognition or TLR-independent induction of inflammatory responses.

A Novel Role for Neutrophils As Critical Activators of NK Cells

Neutrophils are essential players in innate immune responses to bacterial infection. Despite the striking resistance of Legionella pneumophila (Lpn) to bactericidal neutrophil function, neutrophil granulocytes are important effectors in the resolution of legionellosis. Indeed, mice depleted of neutrophils were unable to clear Lpn due to a lack of the critical cytokine IFN-γ, which is produced by NK cells. We demonstrate that this can be ascribed to a previously unappreciated role of neutrophils as major NK cell activators. In response to Lpn infection, neutrophils activate caspase-1 and produce mature IL-18, which is indispensable for the activation of NK cells. Furthermore, we show that the IL-12p70 response in Lpn-infected neutropenic mice is also severely reduced and that the Lpn-induced IFN-γ production by NK cells is strictly dependent on IL-12. However, since dendritic cells, and not neutrophils, are the source of Lpn-induced IL-12, its paucity is a consequence of the absence of IFN-γ produced by NK cells rather than the absence of neutrophils per se. Therefore, neutrophil-derived IL-18, in combination with dendritic cell-produced IL-12, triggers IFN-γ synthesis in NK cells in Lpn-infected mice. We propose a novel central role for neutrophils as essential IL-18 producers and hence NK cell “helpers” in bacterial infection.

TIGIT predominantly regulates the immune response via regulatory T cells.

Coinhibitory receptors are critical for the maintenance of immune homeostasis. Upregulation of these receptors on effector T cells terminates T cell responses, while their expression on Tregs promotes their suppressor function. Understanding the function of coinhibitory receptors in effector T cells and Tregs is crucial, as therapies that target coinhibitory receptors are currently at the forefront of treatment strategies for cancer and other chronic diseases. T cell Ig and ITIM domain (TIGIT) is a recently identified coinhibitory receptor that is found on the surface of a variety of lymphoid cells, and its role in immune regulation is just beginning to be elucidated. We examined TIGIT-mediated immune regulation in different murine cancer models and determined that TIGIT marks the most dysfunctional subset of CD8+ T cells in tumor tissue as well as tumor-tissue Tregs with a highly active and suppressive phenotype. We demonstrated that TIGIT signaling in Tregs directs their phenotype and that TIGIT primarily suppresses antitumor immunity via Tregs and not CD8+ T cells. Moreover, TIGIT+ Tregs upregulated expression of the coinhibitory receptor TIM-3 in tumor tissue, and TIM-3 and TIGIT synergized to suppress antitumor immune responses. Our findings provide mechanistic insight into how TIGIT regulates immune responses in chronic disease settings.

MicroRNA-21 promotes Th17 differentiation and mediates experimental autoimmune encephalomyelitis

Accumulation of IL-17-producing Th17 cells is associated with the development of multiple autoimmune diseases; however, the contribution of microRNA (miRNA) pathways to the intrinsic control of Th17 development remains unclear. Here, we demonstrated that miR-21 expression is elevated in Th17 cells and that mice lacking miR-21 have a defect in Th17 differentiation and are resistant to experimental autoimmune encephalomyelitis (EAE). Furthermore, we determined that miR-21 promotes Th17 differentiation by targeting and depleting SMAD-7, a negative regulator of TGF-β signaling. Moreover, the decreases in Th17 differentiation in miR-21-deficient T cells were associated with defects in SMAD-2/3 activation and IL-2 suppression. Finally, we found that treatment of WT mice with an anti-miR-21 oligonucleotide reduced the clinical severity of EAE, which was associated with a decrease in Th17 cells. Thus, we have characterized a T cell-intrinsic miRNA pathway that enhances TGF-β signaling, limits the autocrine inhibitory effects of IL-2, and thereby promotes Th17 differentiation and autoimmunity.

IFN-γ Limits Th9-Mediated Autoimmune Inflammation through Dendritic Cell Modulation of IL-27

IL-9–producing Th9 cells have been associated with autoimmune diseases, such as experimental autoimmune encephalitis. However, the factors that negatively regulate Th9 cells during autoimmune inflammation are unclear. In this article, we show that IFN-γ inhibits Th9 differentiation both in vitro and in vivo. This suppressive activity was dependent on the transcription factor STAT-1. In addition to its direct inhibitory effect on Th9 differentiation, IFN-γ suppressed Th9 cells through the induction of IL-27 from dendritic cells. In vitro, treatment of naive CD4+ T cells with IL-27 suppressed the development of Th9 cells, which was partially dependent on the transcription factors STAT-1 and T-bet. Moreover, IL-27 treatment completely abrogated the encephalitogenicity of Th9 cells in the experimental autoimmune encephalomyelitis model. Thus, our results identify a previously unknown mechanism by which IFN-γ limits Th9-mediated autoimmune inflammation through dendritic cell modulation of IL-27.

Type‐I IFN drives the differentiation of short‐lived effector CD8+ T cells in vivo

Two subsets of CD8+ T cells are generated early during an immune response; one of these subsets forms the memory pool, known as memory precursor effector cells (MPECs), identified by high expression of CD127 and low expression of KLRG1, whereas the other subset forms short‐lived effector cells (SLECs) identified by low expression of CD127 and high expression of KLRG1. Here, we studied in vivo the role of type‐I IFN in this fate decision. We found that under priming conditions dominated by type‐I IFN, as observed in lymphocytic choriomeningitis virus (LCMV) infection, type‐I IFN signaling directly impacted the regulation of T‐bet and thus the early fate decision of CD8+ T cells. In the absence of type‐I IFN signaling, CD8+ T cells failed to form SLECs but could form MPECs that give rise to functional memory CD8+ T cells. Together, these findings identify type‐I IFN as an important factor driving SLEC differentiation and thus instructing the early division between the effector and memory precursor CD8+ T‐cell pool.